1. Field of the Invention
The present invention relates to a conductive substrate with a resistance layer useful for the production of a resistance circuit board and to a resistance board using that conductive substrate with a resistance layer, more particularly relates to a conductive substrate with a resistance layer and a resistance board capable of maintaining the uniformity of the resistance layer in a step of etching away a high conductivity substrate of the conductive substrate with a resistance layer so as to leave the resistance layer on an insulating board.
2. Description of the Related Art
A printed circuit board material embedding resistors (hereinafter called a “resistance circuit board material”) is generally provided in the form of a multilayer body of an insulating board and a resistance layer adhered to that board and copper foil or other high conductivity bases adhered to that resistance layer.
A printed resistance circuit using a resistance circuit board material is produced by the subtractive method (mask etching method) giving insulating regions where all of the resistance layer and conductive base on the insulating board are removed, resistance regions where the high conductive base is removed, and all remaining conductive regions in accordance with the targeted pattern of the circuit.
In the past, as the material forming the resistance layer, a carbon-based resistance material was generally used. As other methods using metal thin films, electroplated nickel including phosphorus (Japanese Patent Publication (A) No. 48-73762 and Japanese National Publication No. 63-500133), electroplated nickel including tin (Japanese Patent Publication (A) No. 54-72468), etc. have been proposed. With these types of metal thin resistance layers, however, while it is possible to obtain a film with a high sheet resistance by reducing the thickness, in general, if the thickness is reduced, the uniformity of the metal film is lost and a constant sheet resistance cannot be obtained, so there are limits to the reduction of thickness.
That is, in the production of a conductive base with a resistance layer, a thin resistance layer is formed on a conductive base by electroplating, but to raise the bonding strength of the conductive base with a resistance layer to the insulating substrate, the surface of the conductive base is roughened, then is plated with the Ni—P etc. serving as resistance layer. With this method, however, since the resistance layer is present on the rough surface of the conductive base, in particular on the finely roughened surface by fine particles, even right after plating, the uniformity of thickness of the plating is poor and the sheet resistance lacks stability.
Further, dissolution of part of the resistance layer cannot be avoided since the layer of the conductive base is etched away when used as a resistance circuit board material. Further, if there is unevenness in thickness in the Ni—P plated resistance layer, there is the defect that part of the resistance layer is also etched out in order to completely remove the layer of the conductive base. It was extremely difficult to stably leave resistors and produce a printed resistance circuit board. In particular, when roughening treating with copper etc., then plating a resistance layer, the cross-section exhibited knobbed relief shapes due to the roughening with copper and it was difficult to dissolve away all of the copper layer without dissolving the resistance layer. If sufficiently removing the roughening treating layer, the resistance layer was inevitably dissolved and therefore obtaining a stable resistance value was not possible. Further, when producing a multilayer printed resistance circuit board, the printed resistance circuit board is hot pressed. There are therefore the defects that at this time, cracks occur at the portion of only the resistance layer (parts where conductive base is etched away), the resistance increases, or sometimes the circuit becomes open.
In forming a resistance layer by such Ni—P alloy plating, nickel ions, phosphorous acid ions, and phosphoric acid ions are essential. A plating bath for forming the resistance layer also includes sulfuric acid ions and chlorine ions. A conductive base with a resistance layer obtained by plating on a conductive base by using such a bath suffers from ununiform color at the time of plating and variations in the plating layer microscopically. In a wide material used at the time of mass production (for example, having a width greater than 300 mm), variation easily occurs in the plating thickness and phosphorus content in the width direction and the fluctuation in resistance of the resistance circuit becomes greater.
In the case of a resistance layer made of a Ni—Sn alloy, tin oxides or hydroxides remain on the insulating board when the resistance layer is etched to form insulation regions (dissolving the Ni—Sn) and the problem of poor insulation arises. Further, Ni—Cr, Ni—Cr—Al—Si, etc. formed by vapor deposition have been developed for the same purposes, but problems of cost and productivity and also problems of a low bonding strength with the insulating material have been pointed out.